Application Not Working After Target Framework Change

I changed the target framework for my project from 4.5 to 3.5. I had a few errorr messages about the Microsoft.CSharp reference so I changed the framework back to 4.5. Now when I run the application the first form is just dead and doesn't seem to do anything. My first form has a progess bar which is updated using a timer but it doesn't update just stays as it is and nothing happens.

Any ideas?

Regards
Matt